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blk-iocost: fix hweight cache publication and donation fail-safe - #1466
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transfer_surpluses() warns and then keeps going when the donation calculation produced invalid weights. The following propagation can then divide by a zero hweight_inuse, so the warning is immediately followed by a divide error. When the check trips, abandon this period's donation transfer and release the walk lists. This trades one period without donation for not crashing; the invalid state is not expected on the normal path. Developed with assistance from Claude Code (Anthropic): the assistant analysed the failure path, drafted the change and ran the checks listed below. The author reviewed the result and remains responsible for it. Fixes: e08d02a ("blk-iocost: implement Andy's method for donation weight updates") Cc: stable@vger.kernel.org Signed-off-by: Kunhai Dai <daikunhai@didiglobal.com>
current_hweight() publishes the cache generation and both weights with separate stores. Refreshers running on different CPUs can interleave their stores and leave a new generation paired with weights from an older calculation. The compiler can also reorder the generation store before the weight stores, so a reader that sees the new generation can accept stale weights. A stale weight is not only a reporting artifact. It feeds the cost calculation and the donation logic, where an inconsistent combination can lead to a zero divisor in transfer_surpluses(). In addition, the slow path rereads iocg->hweight_active/inuse after publishing them, so the value it returns can be the one written by another refresher instead of the locally computed one. Pack the generation and both weights into a single atomic64_t and publish them with one atomic write. Readers take one atomic snapshot and return the local values. Weights are in [1, WEIGHT_ONE]; storing each minus one preserves the full range in 16 bits and leaves 32 bits for the generation. The two weight fields remain as the donation workspace used under ioc->lock. Tracepoints decode the packed cache without refreshing it and iocost_monitor.py is updated accordingly. Developed with assistance from Claude Code (Anthropic): the assistant drafted the packed representation, the helpers and the tracepoint/monitor changes, and ran the checks listed below. The author reviewed the result and remains responsible for it. Fixes: 7caa471 ("blkcg: implement blk-iocost") Signed-off-by: Kunhai Dai <daikunhai@didiglobal.com>
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Pull request for series with
subject: blk-iocost: fix hweight cache publication and donation fail-safe
version: 2
url: https://patchwork.kernel.org/series/1188903/